Zero Calibration of Combustible Gas Detector

There are no specific standards to define the zero calibration of combustible gas detector. Many analysis processes include some special analysis processes, such as EPA, and use pure nitrogen or purely man-made gas to build the zero point. This is because the bottled nitrogen and the artificial gas can be easily obtained. Due to that, people generally believe that use of bottled nitrogen and man-made gas is a good method for zero-point setting of the sensor.

Unfortunately, this method cannot ensure adequate accuracy. Excluding nitrogen and oxygen, there are other trace gases. Similarly, the surrounding air contains a small percentage of vapor. Therefore, assume that the air in the region is fresh, and to use the surrounding air as the zero point of the sensor holds vital practical significance. The reference point differs based on differences of establishment. Therefore, a good reference point for the sensor within the region always believes that the air within the region, such as certain office region, is fresh. This will obtain a zero point very close to the reality, for it represents the surrounding air conditions. Lack of the vapor might result in that the number of the set zero point is smaller than the zero-point number of the sensor's surrounding air. This is the so-called zero-point drift. One should pay attention to this when the solid-state sensor and the photo-ionization detector are used.

Method for demarcation: Considering all factors, such as the sensor's model and application conditions, we should follow the recommended demarcation methods:

• According to judgment of the operation personnel, the gas nearby the sensor is fresh, and cannot exist normally. At the moment, the indication of the instrument is close to zero (reading). The zero-point setting process can be skipped. If there is any question, the plastic bag can be used to obtain the air nearby the sensor which is believed to be fresh. This is a very fast and easy process, and this method is very effective to distinguish the real alarm from the false alarm.

• A striking advantage of the compressed gas is that the gas can be easily controlled and carried in the bottle. This equipment can easily and conveniently obtain air. In the air, there are few disturbing gases, such as hydro-nitrogen, CO, CO2 and so on. However, the gas has a low moisture, and a coping method is to use the filter with the active carbon in the sampling system to filter out all the undesired gases and vapor from the damp gases. After this, the gas can be used to demarcate different types of sensors. However, of special note is that CO cannot be filtered through active carbons. Therefore, the rule says that the CO content in the bottle should be the same to that in the surrounding ambient gases. Besides, the soda-lime filter can be used to filter out CO. The soda-lime filter can be used to filter out the CO on the sampling system pipeline. This is a good method to set the zero point for the CO2 sensor, which can easily obtain the basic zero point. Though the man-made gases are usually very pure, they cannot be applicable to the solid-state sensor and photo-ionization detector, because sensors of the kind require there should be vapor in the sampled gases. A simple method to address this problem is to use the damp and thin tissue paper on the sampling system pipeline, which can ensure dampness for the sampling current and adequate vapor for the sensor. Besides, NAFION tube can be adopted.